Chapter 6: Electromagnetism

Transcription

1 Physical Science 4011 Electricity Chapter 6: Electromagnetism

2 Review Electricity is simply a collection of electrons. Electrodynamics is the study of electricity that flows through a circuit, under the influence of an EMF, encountering resistance and flowing with a certain current. Electrostatics is the study of electricity that is stationary, giving a charge to its object, and possibly passing to other objects. A magnet is a metal object whose atoms are oriented together. This gives them a north pole and a south pole at opposite ends. Field lines from the north to the south can be drawn that show the force exerted. N S

3 Electromagnetism Interestingly, a moving charge can create the same effect as a magnet. Electricity is a flow of moving electrons, which are charged. Therefore, electrical current in a conductor acts like a magnet. An electrical circuit placed near a compass will pull the needle towards the north end or away from the south end. This is called electromagnetism. Similarly, two circuits with current running in opposite directions are attracted, like opposite ends of magnets. Likewise, two circuits with current running in the same direction are repelled from each other. This effect is far too small to be noticed in household wiring. + - I I I I + -

4 Electromagnetism So how does magnetism in an electrical current work? The current produces a circular magnetic field around the wire. The right-hand rule can remind you how this works. Place your right thumb on the wire in the direction of current. Wrap your fingers around the wire. Your fingers illustrate the direction of the magnetic field lines.

5 Electromagnetism If you loop the wire... The field lines come up, out of the centre of loop... around the outside... then back under the loop... and back up through the centre again. I I

6 Electromagnetism From the side view... We see the field lines coming out of the loop like a fountain. The loop looks like a magnetic field donut. I I The loop behaves like a disk-shaped magnet: N S

7 Electromagnetism If we make multiple loops in the wire, we get a coil. The coil becomes like a pipe for the field lines. The lines flow up the pipe... they cascade out the coil like a fountain... they come back down the outside... and they curve around back into the pipe. I I I I I I

8 Electromagnetism A coiled wire behaves like a bar magnet. Field lines exit the top (north pole), and curve around to enter the bottom (south pole). I I N I I S Page 6.14, exercise 6.9

9 Applications of Magnets Many modern technologies use magnets or electromagnets: Magnetic resonance imaging (MRI) These are medical devices used to create 3D images of the body. It uses the magnetic properties of hydrogen atoms. Information storage Computer hard drives and old-school audio cassettes Small sections of the medium are magnetized north or south (digital signal), or magnetized to varying degrees (analog signal) Speakers Electromagnets vibrate in proportion to the current in the wire. These vibrations are transferred to a membrane, which creates sound. Junkyard lifters Giant electromagnets that attract metal when charged up. They release the metal when the current is shut down.

10 Applications of Magnets Electric motors An electric motor works by the same interaction between a magnet and an electric current passing through a coil. When current (energy) is run through the coil, the magnetic field lines generate a force that spins a magnet. That magnet is the moving piece of the motor. Therefore, the motor converts electrical energy to kinetic energy. Generators Generators are the opposite of motors. When a magnet is forced to spin inside a coil, its magnetic field lines push (induce) a current inside the coil, generating electricity. The magnet is spun by flowing water (hydroelectricity), pressurized steam (thermal or nuclear), air turbines (wind), etc... Therefore, a generator converts kinetic energy to electrical energy.

11 Applications of Magnets Transformers (not Megatron) Recall that transformers convert the voltage and current in an electrical flow without changing the power (V 1 I 1 = V 2 I 2 ). The transformer is made up of two coils......connected by an iron core that runs through each. The core guides the field lines from the primary coil through the secondary coil. V 1 V 2

12 Applications of Magnets Transformers (and not Optimus Prime, either) The voltage conversion depends on the numbers of turns (N) in each coil according to the equation: V 2 = N 2 V 1 N 1 Which can be expanded to: V 2 = I 1 = N 2 V 1 I 2 N 1 V 1 V 2

13 Applications of Magnets Transformers (nor Bumblebee) For example, a transformer has 30 turns in the primary and 150 turns in the secondary. The primary voltage is 20 V. What is the seconday voltage? Use the equation: V 2 = N 2 V 1 N 1 V 1 V 2

14 Applications of Magnets Transformers (and certainly not Starscream) Identify V 1 = 20 V, N 1 = 30, and N 2 = 150 Therefore, V 2 = V 1N 2 N 1 = 20 V 150 turns 30 turns = 100 V V 1 V 2

15 Electromagnetic Interference The Sun frequently has explasions on its surface which send showers of electrons and protons (solar winds) into the planets. These charged particles are mostly deflected by the Earth's magnetic field, so they pose no threat. Occasionally, the solar wind will interact with the Earth's magnetic field to create a light show at the poles called an aurora. At the north pole, it's the aurora borealis. At the south pole, it's the aurora australis. If the solar winds are strong enough, they can penetrate the magnetic field and disrupt electrical transmission lines, leading to large-scale power failures.

16 Electromagnetic Fields and Health The Sun frequently has explasions on its surface which send showers of electrons and protons (solar winds) into the planets. These charged particles are mostly deflected by the Earth's magnetic field, so they pose no threat. Occasionally, the solar wind will interact with the Earth's magnetic field to create a light show at the poles called an aurora. At the north pole, it's the aurora borealis. At the south pole, it's the aurora australis. If the solar winds are strong enough, they can penetrate the magnetic field and disrupt electrical transmission lines, leading to large-scale power failures.